Karns, 26, spent the majority of the 2013 season with Double-A Harrisburg, going 10-6 with a 3.26 ERA (132.2-IP, 48-ER) and 155 strikeouts in 23 starts. He ranked among Nationals minor league leaders in wins (10, tied for fourth), strikeouts (155, second) and ERA (3.26, fourth). Karns also made three starts with the Nationals, including his major league debut on May 28 vs. Baltimore. Following the season, he was ranked by ESPN.com's Keith Law as the No. 4 prospect in the Nationals system, and by Baseball America as the organization's No. 9 prospect.

Over three minor league seasons, Karns is 24-12 with a 2.66 ERA (304-IP, 90-ER) in 60 games (54 starts). He has averaged 10.7 strikeouts per nine innings pitched and opponents have hit only .194 against him. He was named the Nationals Minor League Pitcher of the Year in 2012, going 11-4 with an organizational-best 2.17 ERA (116-IP, 28-ER) and 148 strikeouts in 24 games (18 starts) between Class-A Potomac and Class-A Hagerstown. A 12th-round selection in the 2009 June Draft out of Texas Tech University, he did not pitch professionally until 2011 due to right shoulder surgery.

Lobaton, 29, appeared in 100 games with 76 starts at catcher for the Rays last season and set a club record at the position with a .996 fielding percentage (min. 81 games played). He also set career highs in several offensive categories, including hits (69), doubles (15), home runs (seven), RBI (32), runs scored (38) and batting average (.249). He owns a career .228 average with nine home runs and 52 RBI in four major league seasons, including 2011-13 with Tampa Bay. In Game 3 of the 2013 ALDS vs. Boston, he delivered a walk-off homer off closer Koji Uehara into the Rays Touch Tank, becoming the first Rays player ever to reach the tank and the third player overall since it was installed in 2006.

Rivero, 22, spent the 2013 season with Class-A Charlotte and led the team with career highs in games started (23), innings pitched (127) and wins (nine). He went 9-7 with a 3.40 ERA (127-IP, 48-ER), which ranked sixth in the Florida State League. The lefthander is 29-25 with a 3.45 ERA (386-IP, 148-ER) and 315 strikeouts over five career minor league seasons. Following the season, he was ranked by Baseball America as the No. 17 prospect in the Rays system.

Vettleson, 22, hit .274 (128-for-467) with four home runs and 62 RBI for Charlotte in 2013, leading the Stone Crabs in hits while ranking second in RBI. The former first-round selection (42nd overall) in the 2010 June Draft owns a career .276 batting average, 26 home runs and 171 RBI over three minor league seasons. Following the season, he was ranked by Baseball America as the No. 20 prospect in the Rays system.